This invention relates to the field of circuit interruption devices, more particularly microfuses, and more particularly still arc quenching fillers used to suppress arcing during a microfuse opening cycle.
Microfuses are used to interrupt the electrical circuit path in printed circuits. The microfuse must be physically small, to fit within the geometric boundaries of a circuit board, and be capable of interrupting a circuit in a very short time period to protect delicate transistors and related miniature electronic components. The typical microfuse is approximately three hundred thousandths of an inch long and has a tubular cross-section approximately one hundred and twenty five thousandths thick. To fulfill circuit interrupting functions, the fuse must have arc quenching capabilities.
Arc quenching fuses have been known in the art for many years, and are commonly used in high voltage applications. One such fuse is shown in U.S. Pat. No. 2,007,313, to Sherwood, that discloses a magnesium filler material which performs an arc quenching function. These fillers have a large grain size, typically 25 micron or larger. Such fillers are too large for incorporation into the small envelope of the body of the microfuse. Likewise, they would tend to break the delicate fuse wire used in the microfuse.
Commercial ceramic arc quenching media are available for use with microfuses. One such media used by the Bussmann Division of Cooper Industries, is a combination of silica, magnesia, zirconia and a filler material. One such media is manufactured by Remco Products, Inc. of Ossining, N.Y. The media is applied as a liquid slurry to a ceramic substrate or wafer having a fuse wire attached to opposed metallized areas thereon. The fuse wire may be attached to the metallized areas by ultrasonic bonding, so that the area of the wire between the metallized areas does not touch the substrate. The slurry is allowed to dry, and is then cured in an oven at elevated temperatures to drive off excess water.
This prior art ceramic media has several processing and performance limitations. First, the media tends to crack and shrink during the drying and curing cycles. The shrinkage tends to form voids in the media adjacent the fuse wire. Likewise, the media composition itself appears to create interstitial voids adjacent the fuse wire. Both the cracks and the voids, when adjacent the fuse wire, may lead to premature circuit interruption and undesirable interruption characteristics. Further, the media has insufficient adhesion properties, and thus tends to peel away from the substrate during processing, thereby destroying the fuse.
The prior art ceramic arc quenching media also has limited applicability to inductive circuits. Where a power factor of 94% is encountered, the arc voltage which occurs during fuse opening can be two to three times the rated voltage of the fuse. In fuses employing known ceramic arc quenching media, arcs having enhanced voltages induced by the inductive components of the circuit may cause pressure to build adjacent the fuse wire which is beyond the capability of the media to withstand causing the fuse to explode. No known media for microfuses will yield adequate arc quenching non-catastrophic results in an inductive circuit, i.e., one with a percentage power factor below one hundred percent. The prior art media also exhibits low post-opening resistance, which can allow a leakage current to pass across the open fuse.
The present invention overcomes these deficiencies of the prior art.